Many communication devices such as mobile phones, laptops, or personal digital assistants (PDAs) now are provided with capabilities to allow them to communicate with a wireless local area network (WLAN).
Communication devices may connect with a WLAN upon detection of an access point. To detect access points that are in range, the communication device may actively or passively search for access points having signals that have an acceptable quality, such as having a received signal strength indicator (RSSI) that is above a threshold. A communication device that falls out of the range of an associated access point may attempt to connect with one or more other access points, if available. Since access points are not always readily available, WLAN coverage may be spotty with a mobile device such as a mobile phone, a PDA, etc., and thus mobile communication devices may be continually searching for additional access points with which to connect. For instance, a communication device such as a mobile phone operating in a long range mobile phone network, such as global system for mobile communication (GSM), may continually perform a background scan for WLAN access points, looking for access points having acceptable RSSI levels.
RSSI is generally a measurement of the power of a received signal. Currently, communication devices with WLAN capabilities select access points based on RSSI. For instance, a communication device whose RSSI drops below a certain threshold may seek to connect to another access point with an RSSI above the threshold, if available.
A communication device may scan a wireless communication network that is compliant with the Institute for Electrical and Electronics (IEEE) 802.11 Standard handoff algorithm for available access points using one of two known channel scanning methods, passive scanning and active scanning. Referring to FIG. 1, an example wireless network 100 compliant with the IEEE 802.11 Standard handoff algorithm includes a communication device 102 and a plurality of access points 104-114, each at potentially different distances from the communication device 102. Each access point 104-114 (or at least access points in close proximity or with overlapping coverage areas) may be operating on a different communication channel. To facilitate the discovery of the access points, each access point periodically may transmit a signal often referred to as a beacon to let other communication devices know of its presence. The beacon will include various information including a channel number to indicate which channel it is using. In passive scanning, the communication device 102 may switch to each of a plurality of potentially available channels and listen for a beacon. For each channel, if a beacon is received and an RSSI is above a threshold, the channel/access point may be noted as available. After scanning all the channels, the communication device 102 may choose one of the available access points (if there is more than one) and attempt to associate with the chosen access point. For example, the communication device 102 may choose the access point with the highest RSSI.
When a communication device with WLAN capabilities powers up in an already existing wireless network, it may utilize passive scanning to find available wireless access points and determine with which access point to associate. Passive scanning may also be used when the device is already associated with an access point. For example, if the communication device 102 is a mobile device, it may continuously perform passive scanning in case the connection with the current access point is suddenly lost or degrades. Additionally, if the RSSI for an access point with which the communication device 102 is associated has fallen below a threshold (for example, because the communication device 102 has moved further away from the associated access point), the communication device 102 may utilize passive scanning to find another access point with which to associate. If there are fourteen channels and it is assumed that beacons are transmitted at 100 millisecond (ms) intervals, the device 102 may listen for beacons for approximately 100 ms at each channel, for instance. Thus, the device 102 may spend 1.4 seconds (100 ms/channel×14 channels) scanning for beacons.
In active scanning, the communication device 102 may maintain a list of access points in range of the device 102. The communication device 102 may transmit a probe request signal on each of the channels associated with the access points on the list, except for the access point with which it is currently associated. After transmitting a probe request signal, the communication device 102 may wait a period of time for a response from the corresponding access point. The response may indicate the RSSI of the channel and/or the channel load. Once the communication device 102 has scanned all channels corresponding to the list of access points (except the currently associated access point), the device 102 may seek to disconnect from the current access point and connect to an access point with a higher RSSI or a better combination of RSSI and channel load, for example.